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Mental Disorders And Evolution: What Would Darwin Say About Schizophrenia?

It's a question that's baffled evolutionary theorists for decades: if survival of the fittest is the rule, how have the genes that contribute to serious, debilitating mental disorders survived?

It's been shown that people who suffer from schizophrenia, autism, anorexia and other disorders are less likely to have children. And yet, the genes that cause these disorders aren't going away. In fact, some of the disorders appear to be becoming more common. Evolutionary theory wouldn't predict that.

Scientists have a few theories that attempt to explain this paradox.

One is that the genetic mutations that cause these disorders occurred relatively recently, so not enough generations have passed to allow the evolutionary process to weed them out.

Another theory is that the genetic mutations that cause a disorder in one person somehow make that person's sibling more likely to have children. In a situation like that, the mutation offers a net benefit to a person's family.

A team of Swedish and British scientists recently tested these theories by comparing the rates at which people suffering from mental illness have kids to those of their siblings. The data came from a medical database of more than 2 million Swedes.

The researchers found that the siblings of people who suffer from schizophrenia, autism and anorexia had on average the same or fewer children than the general public, which would seem to confirm the first theory. But they also found that the siblings of people who suffer from depression or substance abuse had significantly more children than the general public, an outcome more in line with the second theory.

We talked with Dr. Peter McGuffin, a professor at King's College London who worked on the study, which was published in the journal Archives of General Psychiatry. Here are highlights from the interview, edited for length and clarity.

Q: You say at the beginning of your paper that "psychiatric disorders have long puzzled researchers by defying the expectations of natural selection." Why?

A: It's particularly the case with schizophrenia, which in this paper and in many other papers has been shown to be a disorder that drastically reduces your fecundity — the number of kids you have. It's often referred to as reduced fertility but, strictly speaking, people with schizophrenia aren't infertile. It's just that they're less often likely to find a partner and have kids.

Schizophrenia is estimated to have a heritability of around 80 percent. Same is true for autism. So if these disorders are very heavily influenced by genes, but the people who have the disorders are less likely to pass on their genes, why aren't the disorders becoming less common in the population?

Q: What are some of the theories as to why this might be going on?

A: There are other gene disorders that have selection pressures against them, but are maintained in the population. A brilliant example of that is sickle cell disease. If you have sickle cell disease, the chances are if it's untreated it's going to kill you before you reach early adult life. Whereas, if you have the sickle cell trait — which is to say, you have one copy of the gene, not two copies — it protects you against malaria if you happen to live in an area where malaria is rife. So there's a selective disadvantage to having the disease, but there's a selective advantage to having the trait.

Q: Your study looked at not just people who were affected by psychiatric disorders, but also their siblings. Why?

A: The hypothesis would be that the relatives of the people who have the disorder, who don't actually have the disorder themselves, are compensating by having more children. I mean, not deliberately compensating by going out and having more children, but there's just something about their makeup that makes them have more kids.

So that's essentially what we were testing in this paper. We were looking at the fecundity of schizophrenics, which we found to be low, as was the fecundity of people with autism. The question is, do their relatives actually make up for this by having more kids because they're advantaged in some way? And the answer is no in the case of schizophrenia, but yes in the case of depression.

Q: If I'm someone who is the sibling of someone who has a psychiatric disorder, what do I need to know? Do I need to think twice about having biological children?

A: You need to know you'll have an increased risk of getting the disorder yourself compared with the general population. The risk in siblings is increased, but it's not increased so dramatically that it ought to stop you from having kids.

These aren't single-gene disorders. These are complex disorders where being a relative is just a risk factor, not a certainty factor.

Q: So ultimately, if I'm the sibling of someone who has one of these disorders, I should be aware of the risks, but it's not something that would make me say "I'm not going to have children."

A: It shouldn't make you say that. And if you are particularly concerned about it and you have more than one relative affected by it in the family, it might be worth seeking [genetic] counseling from an expert who knows what the risks are. [Copyright 2012 National Public Radio

There is no actual 'purpose' to evolution in the sense of seeking some ultimate, perfected form that is the 'highest form' of some organism. There IS no 'highest form' of an organism. Evolution is simply a response that physical systems make to environmental systems. Natural selection doesn't have some 'ultimate perfection'' to it - it's simply a fact of how systems interact.

As an example, take the development of various horse species. Millions of years ago, horses had little squishy toenail-toes like tapirs, and were the size of fox terriers. Ate leaves, lived in swamps. As the environment changed, animals with slightly harder feet more often survived. Mutations occured and they were often advantageous and were 'selected for'. On large expanses of the earth, the swamps dried up and were replaced with dry land and grasses. Horses were selected for more hoofy like toes.

If today, all the grasslands turned into swamps, the horse would once again be 'selected for' little squishy toenail toes like the tapirs, and the type would change again.

Of course it's more complicated than that.

Some mutations occur and are either insignificant in the environment the creature lives in, or have a negative effect.

But keep this in mind. Schizophrenia usually develops in the teens and early twenties. There were times in human existence when 'the early twenties' was probably...OLD. After one had had children, built one's nest and been a provider. There may have been absolutely no evolutionary pressure on it at all at points in human history.

And again - the low fecundity of schizophrenics is seen by many as an artifact of the state hospital system, NOT some innate trait of the disorder. One thing I have learned in my life is that mental illness, even severe mental illness, is rarely a barrier to a person having children.

Especially for women, who often develop schizophrenia 10 or more years later than males do. When most men choose a mate and marry and even father their first and second (or more) children in their late teens, a great many schizophrenic women will have absolutely no symptoms as yet.

Even for men - who more often become ill sooner in life.

Same with menopause. Once a female has reproduced however many times, if a disorder like menopause occurs that doesn't happen til well after that process is over, or usually doesn't develop til long after most females have died, there's no evolutionary pressure on that trait at all.

Again, it's all about the environment the individual is in. It is the environment that makes the environmental pressure, and it is actually very difficult to know how past environments interacted with mental illnesses and their occurence.

For example, it's very difficult for me to imagine that a simple agrarian society really would 'select against' the bulk of people with schizophrenia - who are, after all, mildly affected anyway.

Quoting momtoscott:

Interesting article and even more interesting responses. Thanks for posting.

One thing that annoys me is when people attribute a "purpose" to evolution and then say something undercuts that (often as some kind of argument for a creator with a better plan). A set of cumulative effects does not necessarily indicate any underlying intent!

my son always had issues it's not easy to deal with he hears voices and talks to them he can't process his emotions like normal people he's socially awkward the list goes on most people think people with this are psycho killers there not. my son takes 2 meds intuniv and his rispidone for the Sctz

Quoting lancet98:

I think the best takeaway message parents can get from the original article that started the thread, is that not everybody writing about schizophrenia, really has much understanding of it through history or offers much constructive information. Most who write about it have some very misguided ideas. My best source of information has been the very people who have this illness. And since each person is so individual in how the illness affects them, each person with the illness, has something to teach us.

For a GOOD read, try Mueser and Gingerich's 'COPING WITH SCHIZOPHRENIA'. A great, positive and HELPFUL book! Communication and understanding really is the key....not musing about what Darwin would have said about it, or why it still exists.

No I am not into the idea that ppl have harm in mind, I am disagreeing with MYTHS. And your posts are overflowing with myths, misapprehensions and misdirections that are the result of pointless uninformed speculation that you have soaked up and swallowed as utter and complete gospel.

Stupid idiotic myths that people who know NOTHING about mental illness or developmental delays have spread INSTEAD of providing useful information.

And that article doesn't represent any CONSENSUS at all, it's just another stupid, ridiculous MYTH.

Hundreds of years ago neurologists were doing autopsies on human brains and documented very, very thoroughly, the brain changes that occur in schizophrenia. These are frank and obvious and it has been understood for a very long time that these are actual brain diseases that affect specific parts of the brain.

Exercise some critical reading skills for God's sake.

Concrete thinking is no more a characteristic of 'primitive people' than a shoe is characteristic of a chicken.

And the idea that 'primitive people' have some sort of different thought process is absurd. People who live in tribal societies are just as intelligent as the guy who works in an office or - God forbid - teaches psychology.

Probably far more skilled and adept at a great many things. Many SO CALLED 'primitive' people have encyclopedic knowledge of the environment, seasonal changes, habits of animals, growth patterns of plants and trees, weather conditions, together with having a complicated, multifaceted culture that includes a GREAT DEAL of non-concrete thinking, using all sorts of thought processes and patterns. Furthermore, most 'intelligent' 'civilized' people don't have a snowball's chance in hell of learning the skills many of these 'primitive' people use - when researchers or tourists visit them and try to do things they do, they do nothing but get in the way and bollix up the works. As one example, drummers in one African tribal group, traditionally learned an entire drum pattern consisting of multiple layered patterns, from start to finish, without breaking it down into steps, or practicing individual patterns. Also, people traditionally in many groups learned extensive complex oral traditions.

There is NOTHING that is 'primitive' about 'primitive' people. In the past? Oh baloney. Fifty thousand years ago your 'primitive man' that left settlements in France, had plumbing, water storage, settlements with private and communal areas, and a complex language that researchers have been attempting to model with computer science.

The idea that there even EXISTS 'primitive people' is stupid and ethnocentric and racist.

The idea that concrete thinking is some sort of 'retreat to a primitive manner of thinking' is LAUGHABLE. It's truly an idiotic premise.

It's a stupid, ridiculous idea that comes with an embedded ethnocentrism, racism and a refusal to get acquainted with even the most SIMPLE BASIC FACTS about schizophrenia, psychosis, neurology and mental illness in general. You're trotting out nonsense that was debunked as laughable and irrelevant and culture-centric garbage, MANY years ago.

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